🔗 Lap Joint Efficiency Calculator
How much of your overlap is actually carrying load. Enter the joint and see where the shear piles up, and the point past which a longer overlap stops buying anything.
📐 The joint
The assumption worth questioning
Our lap joint designer sizes an overlap the standard way: area equals load divided by shear strength, so overlap equals load divided by shear times width. Every adhesive datasheet is written to be used like that, and it is the right way to start.
It contains one assumption that is never true: that the shear is the same everywhere along the bond. It is not. The adherends stretch under load, each carrying everything at one end and nothing at the other, and the adhesive has to accommodate the difference. That accommodation is concentrated at the two ends of the overlap. The middle of a long joint is close to idle.
Which gives the practical rule this page exists for: length has a limit, width does not. Doubling the overlap adds area where the stress already is not; doubling the width adds area everywhere the stress actually is. If a joint will not carry the load, widen it, thicken the adherends, or taper the overlap ends — and only then make it longer.
❓ Frequently Asked Questions
Why isn't the shear the same all along the overlap?
Because the adherends stretch. Each one carries the full load at one end of the joint and none at the other, so along the overlap they stretch by different amounts. The adhesive layer has to take up that difference, and the mismatch is greatest at the two ends — so that is where the shear piles up, falling away to very little in the middle. Volkersen set this out in 1938 and it is the single most useful thing to know about a bonded joint.
So doubling the overlap doesn't double the strength?
No, and past a point it barely helps at all. The extra length lands in the middle of the joint, which is the part already doing the least work. Past roughly twenty times the adherend thickness you are bonding area that will never be meaningfully stressed. If you need more capacity, add WIDTH — which scales properly — or use thicker adherends, which reduces the stretch mismatch in the first place.
How does this relate to the lap joint designer?
The designer sizes an overlap from load, adhesive shear strength and joint width. That calculation assumes the average shear is carried everywhere, which is the right first cut and is how overlaps are specified. This page tells you how much of that average the joint actually reaches, so you can see when the simple sizing is comfortable and when it is optimistic.
Is the efficiency figure a safety factor?
No, and it should not be used as one. It is the elastic result, which assumes the adhesive never yields — the pessimistic end. A ductile structural adhesive yields at the overlap ends and hands load back to the middle, recovering much of the loss; a brittle adhesive does not. Take the number as the shape of the problem rather than as a design allowable, and test a representative joint before relying on any of it.
What actually breaks a single-lap joint?
Usually peel, not shear. The same end effects that concentrate the shear also pry the adherends apart at the overlap ends, and adhesives are far weaker in peel than in shear. This page does not model peel at all. Practically, tapering or filleting the ends of the overlap does more for a real joint than making it longer — it spreads the concentration rather than adding area that cannot be used.